1. Technical Field
The present disclosure relates to a method and device capable of measuring the thicknesses of various kinds of multilayer films.
2. Related Art
FIG. 5 shows the structure of a film thickness meter that measures the thickness of a multilayer film. In FIG. 5, white light emitted from a light source 11 is provided to a multilayer film 10 through an optical fiber 12. The light reflected by the multilayer film 10 is guided to a spectroscope 13 through the optical fiber 12.
The spectroscope 13 disperses the reflected light, converts the dispersed light into an electric signal, and generates a spectrum. The spectrum is acquired by a spectral data acquiring unit 14 and is then output to an optical thickness calculator 15.
A setting unit 16 sets a wavelength band for measuring the optical thickness and a peak detection range for detecting the peak of a power spectrum.
Frequency interference fringes that are proportional to the optical thickness appear in the spectrum. The optical thickness calculator 15 measures the interference fringes and calculates the optical thickness.
For the wavelength band set by the setting unit 16, a reflectance spectrum is converted into a wavenumber domain reflectance spectrum in which the wavelength is arranged at equal intervals. Then, Fourier transform is performed on the data of the wavenumber domain reflectance spectrum to calculate the power spectrum in the set wavelength band. The peak of the power spectrum is detected in the peak detection range set by the setting unit 16. The optical thickness is obtained from the position of the peak.
The optical thickness calculated by the optical thickness calculator 15 is input to a physical thickness calculator 17. The physical thickness calculator 17 calculates the physical thickness, which is the actual film thickness, of each layer of the multilayer film 10, based on the optical thickness and the refractive index. The physical thickness is displayed on a display unit 18.
Next, the relationship between the optical thickness and the physical thickness will be described with reference to FIG. 6. FIG. 6 is a cross-sectional view illustrating the multilayer film. The multilayer film includes two layers 20 and 21, and the physical thicknesses of the two layers 20 and 21 are d11 and d12, respectively. The layer 21 is a film and the layer 20 is a coating layer formed on the film 21. The physical thicknesses d11 and d12 of the layers 20 and 21 are 1 μm and 150 μm, respectively.
It is assumed that white light enters from the layer 20 so that the film thickness is measured. The light is reflected from the boundary surface between the layers 20 and 21 and the rear surface of the multilayer film, and the optical thickness is obtained from the reflected light. The optical thickness obtained from the light reflected by the boundary surface between the layers 20 and 21 is L11 and the optical thickness obtained from the light reflected by the rear surface of the multilayer film is L12. The optical thickness L11 relates to only the layer 20, but the optical thickness L12 relates to both the layer 20 and the layer 21.
Ideally, it is possible to measure the optical thicknesses corresponding to the number of combinations of the boundary surfaces of the multilayer film. However, since the optical thickness that can be actually measured depends on the reflectance or flatness of each boundary surface, it is not necessarily possible to directly detect the optical thickness of the desired layer.
The optical thickness is the product of the physical thickness and the refractive index. Therefore, when the optical thickness of the desired layer can be directly measured, it is possible to calculate the physical thickness with a simple operation. However, when it is difficult to directly measure the optical thickness of the desired layer, an expression for obtaining the physical thickness needs to be derived from a combination of the optical thicknesses of a plurality of layers.
JP-A-2008-292473 discloses an example in which the optical thickness is measured in a plurality of wavelength ranges using the relationship between the wavelength and the refractive index depending on the wavelength, which makes it possible to independently measure the physical thickness of each layer even though a plurality of layers have the same film thickness. The above-mentioned example will be now described with reference to the multilayer film shown in FIG. 6.
When the refractive indexes of the layer 20 in the wavelength ranges W1 and W2 are n11 and n12 and the refractive indexes of the layer 21 in the wavelength ranges W1 and W2 are n21 and n22, the following Expressions (1) and (2) are given. Also, the optical thickness L12 is the sum of the optical thicknesses of the layers 20 and 21:
                              d          ⁢                                          ⁢          11                =                  L          ⁢                                          ⁢                      11            /            n                    ⁢                                          ⁢          11                                    (        1        )                                                                                    d                ⁢                                                                  ⁢                12                            =                                                                    (                                                                  L                        ⁢                                                                                                  ⁢                        12                                            -                                              d                        ⁢                                                                                                  ⁢                        11                        ×                        n                        ⁢                                                                                                  ⁢                        12                                                              )                                    /                  n                                ⁢                                                                  ⁢                22                                                                                        =                                                                    (                                                                  L                        ⁢                                                                                                  ⁢                        12                                            -                                              L                        ⁢                                                                                                  ⁢                                                  11                          /                          n                                                ⁢                                                                                                  ⁢                        11                        ×                        n                        ⁢                                                                                                  ⁢                        12                                                              )                                    /                  n                                ⁢                                                                  ⁢                22                                                                        (        2        )            
As shown in FIG. 6, d11 and d12 are the physical thicknesses, and L11 and L12 are the optical thicknesses. The optical thickness L12 relates to both the layer 20 and the layer 21. Therefore, d11 needs to be calculated by Expression (1) and d11 needs to be substituted into Expression (2) to calculate d12.
It is possible to calculate the physical thicknesses d11 and d12 from Expressions (1) and (2) and the measured values of the optical thicknesses L11 and L12.
However, the film thickness meter has the following problems. Expressions (1) and (2) depend on the optical thickness that can be measured or the structure of the multilayer film to be measured. However, it is difficult for the user to know in advance a difference in the refractive index due to the wavelength and derive an expression for calculating the physical thickness from the optical thickness. Even when the user can derive the expression, it is difficult to prepare a structure in which the film thickness meter receives any type of arithmetic expression.
In order to solve the above-mentioned problems, all expressions for calculating the physical thickness from the optical thickness need to be installed into the film thickness meter in advance. However, since it is difficult to install all arithmetic expressions, the individual arithmetic expression needs to be installed in accordance with each customer's request. Therefore, it takes a lot of time and effort to calculate the physical thickness.
In addition, since it is necessary to measure a multilayer film of a different brand after the device is delivered, it is difficult for the user to respond to the circumstances, and the film thickness meter needs to be redesigned.